Chevy Malibu running hot

I have a chevy malibu 2002 80000miles. My car was running hot. I took my car to an mechanic and he told me I needed a new thermostat. I told him to replace the thermostat. When I went to pick up my car the check engine light was on, oil was leaking, the radio was turning on and off. When I asked him what was wrong with my car he was supposed to fix it. He said an head gasket is blown. When I took the car to him no lights was on the dash there was nothing leaking and no power failure. This mechanic is trying to tell me since the thermostat was broken I didnt have any symptoms of an blown head gasket, but since he fixed it thats when you can tell. Is this possible or did he mess up something. When I took it in it was just running hot now my check engine light is on and oil is leaking. Please help me